Parity (childbearing) significantly decreases a woman’s risk of breast cancer. Several factors within the mammary gland are postulated to contribute to parity-induced protection including (but not restricted to), a reduction in the number of mammary stem cells (MaSCs). Several groups over the past decade have investigated whether there is a decrease in MaSC with parity but have yielded conflicting results. The discord in results is most likely a reflection of the different methods used to analyse MaSC changes including the procedure used to dissociate mammary gland tissue and isolate MaSCs, variations to testing the in vivo repopulating activity of MaSCs and even the mouse model of parity used. In an effort to understand the difference in results obtained by different groups we have compared the in vivo repopulating activity of unfractionated mammary cells and FACS isolated MaSC enriched populations from parous and age-matched nulliparous animals. We have also performed transplantation of non-MaSC enriched FACS isolated epithelial populations. Our results show a 3.4-fold decrease in stem cell activity in non-fractionated epithelial cells from parous animals, however we were unable to show a decrease in primary or serial in vivo repopulating activity in FACS isolated MaSCs. There is a significant decrease in repopulating activity in non-MaSC enriched cells from parous animals. This indicates that parity does not decrease stem cell activity of the FACS enriched MaSCs but does decrease stem cell activity in non-stem cell enriched populations.